This invention relates to a member applied as a motive power transmission part requiring a high degree of surface fatigue strength in the manner of gears and bearing rolling elements, and more particularly, to a high bearing pressure-resistant member and a production process of such a high bearing pressure-resistant member that is suitable for use under high bearing pressure in semi-high-temperature to high-temperature environments (of roughly within a range of from 100 to 300xc2x0 C.).
A motive power transmission part of the prior art like that described above is used after subjecting to surface hardening treatment such as carburization and carbonitriding having for its base material (matrix) mechanical structure steel represented by SCr420H steel (chromium steel) and SCM420H steel (chromium-molybdenum steel) defined in JIS G 4052 (structural steel materials having guaranteed quenching).
However, accompanying the higher engine outputs and reduced size and weight of parts used in, for example, automobiles in recent years, the loads being applied to motive power transmission parts is tending to increase, and there are a growing number of cases of use under semi-high to high temperatures (roughly 300xc2x0 or lower) and under high bearing pressure.
Although there is, for example, the method of high-density carburization treatment, which improves temper softening resistance as a result of increasing hardness by aggressive precipitation of Fe3C (cementite), as a method of increasing surface fatigue strength of such parts, cementite is susceptible to precipitation in the form of a coarse reticular form along the grain boundary during carburization, and the coarse carbide (cementite) precipitated in reticular form near the grain boundary results in quench cracking that not only lowers tenacity, but also decreases rolling fatigue strength.
On the other hand, there are also methods of precipitating carbide using steels containing Cr, Mo, V or W as in AMS64900 used at semi-high to high-temperature ranges. Although these methods result in improvement of pitting resistance and rolling fatigue service life at semi-high to high-temperature ranges, since these steels contain large amounts of alloy elements, in addition to the cost of the base material (matrix) becoming high, there are also problems including decreased ease of cutting, and the resolution of such problems has been a topic for high bearing pressure members of the prior art.
It is an object of the present invention to provide an improved high bearing pressure-resistant member and a process of producing the same member, which can effectively overcome drawbacks encountered in conventional similar techniques.
Another object of the present invention is to provide an improved high bearing pressure-resistant member and a process of producing the same member, which high bearing pressure-resistant member has excellent surface fatigue strength such as pitting strength and rolling fatigue strength even under semi-high to high temperatures and high bearing pressure such that local bearing pressure exceeds 3 GPa, while also inhibiting increases in base material cost and decreases in cutting ease caused by the addition of large amounts of alloy elements in comparison with AMS64900 of the prior art, and not requiring complex heat treatment.
An aspect of the present invention resides in a high bearing pressure-resistant member which is formed of a mechanical structural steel containing Cr. The mechanical structural steel includes a matrix having at least one of martensite structure and bainite structure. The matrix contains carbide having a mean particle size of 3 xcexcm or less, dispersed and precipitated in form of at least one of generally spheres and pseudo-spheres.
Another aspect of the present invention resides in a process of producing a high bearing pressure-resistant member.
The production process comprises (a) performing carburization treatment on a material member formed of a mechanical structural steel containing Cr so that the material member has a surface carbon density within a range of from 0.6 to 1.5% by weight; (b) precipitating carbide by holding the carburized material member at a temperature having an upper limit temperature T (xc2x0 C.) which is calculated from an equation: T=675+120xc2x7Si(wt %)xe2x88x9227xc2x7Ni(wt %)+30xc2x7Cr(wt %)+215xc2x7Mo(wt %)xe2x88x92400xc2x7V(wt %); and (c) quenching the carbide-precipitated material member by rapidly cooling the carbide-precipitated member after holding the carbide-precipitated member at a temperature at which austenite phase is formed.
A further aspect of the present invention resides in a process of producing a high bearing pressure-resistant member. The production process comprises (a) performing a carburization treatment on a material member formed of a mechanical structural steel containing Cr so that a surface carbon density of the mechanical structural member is within a range of from 0.6 to 1.5% by weight; (b) precipitating carbide by holding the carburized material member at a precipitation temperature Tp (xc2x0 C.) having an upper limit temperature T (xc2x0 C.) calculated according to a first equation: T=675+120xc2x7Si(%)xe2x88x9227xc2x7Ni(%)+30xc2x7Cr(%)+215xc2x7Mo(%)xe2x88x92400xc2x7V(%) based components of the material member for a time shorter than a time t (hr.) calculated according to a second equation: t=10{19000/(Tp+273)xe2x88x9220} based on the precipitation temperature Tp (xc2x0 C.); and (c) quenching the carbide-precipitated material member by rapidly cooling the carbide-precipitated member after holding the carbide-precipitated material member at at least one of a Ac1 transformation temperature and a temperature (austenite region temperature) at which austenite phase is formed.